November 1, 2013

Vital Precipitation Could Be Lost Through Climate Geoengineering

A new study, led by scientists from the National Center for Atmospheric Research (NCAR), shows that although a significant build-up in greenhouse gases in the atmosphere would alter worldwide precipitation patterns, a widely discussed technological approach to reduce future global warming would also interfere with rainfall and snowfall.

The findings, published online in the Journal of Geophysical Research: Atmospheres, demonstrate that global warming caused by a massive increase in greenhouse gases would spur a nearly 7 percent average increase in precipitation compared to preindustrial conditions.

"Geoengineering," however, could result in a 5 to 7 percent decrease in monsoonal rains in North America, East Asia, and other regions compared to preindustrial conditions, according to the researchers. Average precipitation worldwide could decrease by approximately 4.5 percent.

“Geoengineering the planet doesn’t cure the problem,” says NCAR scientist Simone Tilmes. “Even if one of these techniques could keep global temperatures approximately balanced, precipitation would not return to preindustrial conditions.”

With concerns rising over climate change, researchers have studied geoengineering approaches aimed at reducing future warming. Some of these methods would capture carbon dioxide before it enters the atmosphere, while others would attempt to essentially shade the atmosphere by injecting sulfate particles into the stratosphere or launching mirrors into orbit with the goal of reducing global surface temperatures.

The current research focuses on the shading method. The researchers caution, however, that the planet's climate would not return to its preindustrial state even if the warming itself were mitigated.

“It’s very much a pick-your-poison type of problem,” says NCAR scientist John Fasullo. “If you don’t like warming, you can reduce the amount of sunlight reaching the surface and cool the climate. But if you do that, large reductions in rainfall are unavoidable. There’s no win-win option here.”

A team of scientists from NCAR and 14 other international organizations collaborated on this study, which used the NCAR-based Community Earth System Model, among other tools.

To simulate global precipitation patterns in the event of carbon dioxide levels in the atmosphere reaching four times the preindustrial level, the team used 12 of the world's leading climate models. These quadrupled levels, which is on the high side of projections for the end of this century, were chosen for simulation in order to clearly draw out the potential impacts of geoengineering. They also simulated global precipitation patterns if the incoming solar radiation were reduced.

The team found an increase in carbon dioxide levels would significantly increase global average precipitation, which is in line with other studies. They caution that there would still likely be significant regional variations and even prolonged droughts in some areas.

The reason for the increased rain and snowfall largely has to do with increased evaporation, which would pump more moisture into the atmosphere as a result of more heat being trapped near the surface.

The researchers took their investigations a step farther, examining what would happen if a geoengineering approach partially reflected incoming solar radiation high in the atmosphere.

They found that the amount and frequency of precipitation would decrease significantly, especially for heavy rain events. These effects would be greater over land than ocean, and especially pronounced during months of heavy, monsoonal rains. In the simulations, monsoonal rains dropped by an average of 7 percent in North America, 6 percent in East Asia and South America, and 5 percent in South Africa. The decrease was just 2 percent in India, however. In Western Europe and North America, heavy precipitation dropped even more during the summer.

Two primary reasons help explain this reduced precipitation.

The first involves evaporation. Less water vapor would be pumped into the atmosphere through evaporation as Earth is shaded and less solar heat would reach the surface of the planet.

The second reason involves plants. Plants partially close their stomata when more carbon dioxide is present in the atmosphere. The stoma is the opening that allows a plant to take in carbon dioxide while releasing oxygen and water into the atmosphere. Partially closed stomata would release less water, which would cause the cooled atmosphere to become even drier over land.

The study did not address such issues as how certain crops would respond to a combination of higher carbon dioxide and reduced rainfall.

“More research could show both the positive and negative consequences for society of such changes in the environment,” she says. “What we do know is that our climate system is very complex, that human activity is making Earth warmer, and that any technological fix we might try to shade the planet could have unforeseen consequences.”